This invention relates, in general, to semiconductor devices, and more particularly to a method of fabricating high switching speed diodes and transistors in which the manufacturing yields are improved.
It is known that doping semiconductor devices with near mid band-gap life-time killing impurities results in faster excess carrier recombination during turn-off of the device such that the switching speed is correspondingly increased. Gold and platinum, for example, are used as impurities or dopants and act as a trap for minority carriers which results in increasing the switching speed of a diode or a transistor.
Typically, high speed switching transistors are fabricated by manufacturing a transistor on one side of a silicon wafer and depositing the metal dopant on the other side. The metal dopant, for example gold, is driven into the transistor by placing the wafers into a furnace tube with a temperature of approximately 950 degrees Celsius for about 30 minutes. In order to achieve high switching speeds, it is important that the gold is evenly distributed through the base region. To accomplish this, the gold is evenly distributed throughout the emitter, base and collector regions. However, it is a well known fact that gold would rather segregate to the N-type regions than the P-type regions as the wafer cools. It is for this reason that when the wafers are withdrawn from the furnace tube, the wafers are subjected to a vapor from liquid nitrogen. The vapor quick cools the wafer and prevents the gold from migrating out of the P- base region and into the N+ emitter region and the N-type collector region. Unfortunately, the high furnace temperature followed by the fast cool down cycle causes many of the wafers to break or warp. This is due to the nonuniform cool down cycle which creates thermal stress within the wafer.